Saturday, August 20, 2011

Anthropogenic Global Warming

My review. Additions and clarifications welcome.
The Argument:
A. Human activities put a lot of CO2 in the atmosphere
B. The resulting increase in CO2 is large
C. More CO2 makes the Earth warmer
D. The effects are potentially dramatic (and mostly bad)

Why it’s Complicated:
A. The carbon cycle is complicated: there are several sources and reservoirs.
B. Radiative balance is complicated. Many factors are in play and they interact. Several different gases contribute to the greenhouse effect.
C. Calculation of the size of the effect is difficult, because feedbacks play a key role.
D. Many aspects of the impact of climate change are uncertain.

Kinds of Evidence:
A. Measurements of CO2 produced and atmospheric CO2
We know two central facts: Human activities are putting a lot of carbon (CO2) into the atmosphere and the amount of atmospheric CO2 has increased dramatically since the widespread use of fossil fuels began.
B. Physics of Radiation
a. A relatively simple direct calculation shows that the Earth is a good deal warmer than it would be without greenhouse gases (33 degrees C = 59 degrees F warmer.)
b. Another simple calculation shows the first order contribution (without feedbacks) of the additional CO2 that we have seen to date.
c. The question of feedbacks remains complicated, with mainstream climate science pretty uniformly believing that feedbacks are fairly large and positive, but not all doubters yet rounded up and shot. Convinced.
C. The temperature history of the Earth
a. Surface measurements for 180 years or so.
b. Satellite measurements for about 30 years
c. Several different kinds of inferred temperatures with different precisions and uncertainties. Most of these are very technical, but qualitative indications (glaciers covering Europe, or alternatively, plant life growing abundantly in the Arctic ocean) show that the planet has been several degrees colder at times and several degrees warmer at others.
D. Measurements of past CO2 concentrations. Again, very technical. Several different methods with varying precision. Again, the data show that CO2 levels have been a good deal lower during glacial episodes and (with less certainty) a good deal higher during at least one prior hot period.
E. Numerical Climate Models. These extremely complicated models are among the largest and most elaborate computer calculations ever done. The steady increase we have seen in computer power has allowed these to become ever more detailed and complete in their modeling of the climate system, but significant uncertainties remain. We should expect them to continue to improve. Such skill as they have is purely statistical – they can’t yet reliably predict year to year or even decade to decade climate changes.

Possible Complicating Factors
A. Long Period Oscillations
B. Solar Output
C. Cosmic Effects